There are numerous devices which displace and/or expand fluids, both gaseous and liquid. Such devices have been implemented in both piston and rotary apparatus. They have been implemented within the same drive line and sometimes on the same shaft. However, the compression and expansion functions have not been implemented within the same device.
The present invention builds upon the technology of U.S. Pat. Nos. 4,907,950, 5,004,404 and 5,114,321, all of which are incorporated herein by reference. Those patents describe a variable positive fluid displacement device comprised of two sets of piston pairs operating in a rigidly connected configuration, each piston pair at 90 degrees to the other. The above-referenced patents have been used as a compressor and also as an expander and in either a 2 or 4 piston configuration. All pistons are operated from a common drive shaft and the pumping/compression or vacuum/expansion of each chamber port has a different peak position in time as a function of the position of the chamber valving which is also driven and controlled by shaft rotation.
The variable positive displacement device of the above-referenced patents can be used to benefit those fuel cell systems in which compressed air is used as an oxygen source. A first such device is used as a compressor with the variable displacement feature used to provide control over fuel cell power output. For vehicular use such control acts as an accelerator to provide a desired transient response. By using a second variable or fixed displacement device as an expander, the overall efficiency of the system can be improved. As an expander, it will return power to the system as a function of the amount of energy available to its intake stream and its recovery efficiency. This now has the desirable result of lowering the power consumption for the compressing function. These concepts are discussed in the publication, incorporated herein by reference, "A Variable Displacement Compressor/Expander for Vehicular Fuel Cell Air Management," SAE International, 400 Commonwealth Drive, Warrendale, Pa. 15096-0001. U.S. Pat. No. 5,434,016 discusses the use of a rotary compressor in a fuel cell system with a variable capacity expander, both compressor and expander preset to desired values.